The Fracture Mechanism of As-Cast and Extruded SiCp/AZ91 Composites Fabricated by Stir Casting

Abstract:

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The fracture mechanisms of SiCp/AZ91 composites were investigated by scanning
electron microscopy (SEM). For the as-cast composites, the decohesion at SiCp/matrix interface is
the main fracture mechanism because of the high stress concentration resulting from the segregation
of particles in grain boundaries formed during solidification process. But for the extruded
composites, the main fracture mechanism is the particle crack or ductile rupture of the matrix
between the particles. So the fracture mechanism of SiCp/AZ91 composites is altered by extrusion
because the segregation of particles and defects in the grain boundaries are largely eliminated by
extrusion.

Abstract: Aluminum borate whiskers and magnetic ceramic particles (NiFe2O4 or Fe3O4) reinforced
aluminum matrix composites were synthesized by a squeeze casting technique. Microstructures were
observed using scanning electron microscopy (SEM). The tensile strength and tensile yield strength of the
as-cast composites containing NiFe2O4 particles were higher than that of composites containing Fe3O4
particles. The tensile yield strength of the thermomagnetically treated composites was higher than that of
the as-cast or the thermally treated composites. The effects of the magnetostrictive property of Fe3O4 or
NiFe2O4, the interface and the thermal residual stresses on tensile behaviors of the composites with
different treatments were discussed in this paper.

Abstract: In this study, the relationship between microstructures and mechanical properties of the extrusion processed Al-15wt.%Mg2Si composite was investigated after applying various extrusion ratios (6:1, 12:1 and 18:1) and solution treatment. Various techniques including metallography, tensile testing and SEM fractography were utilized to characterize the mechanical behavior of the MMC. Results demonstrated that extruded and heat treated composite possesses considerably higher strength and enhanced ductility in comparison with the as-cast samples. It was also found that heat treatment and extrusion processes do not change the primary Mg2Si morphology considerably, but its size increases as extrusion ratio decreased. Heat treatment and extrusion ratio effects on tensile strength, elongation of extruded specimens were also studied in this work.

Abstract: Ti-6Al-2.5Sn-4Zr-0.7Mo-0.3Si-0.3Y alloy matrix composite reinforced with 7%vol TiB whiskers was fabricated by Induction Skull Melting (ISM) technique and one-direction forged technology utilizing the reaction between titanium and TiB2. The microstructure, room temperature and high temperature mechanical properties have been presented and discussed. Microstructural analysis of the composites revealed that the microstructure was significantly refined and TiB whiskers were made to align the longitudinal direction after forging. It shows that the tensile strength and ductility of the composites has a significant improvement, especially at room temperature.

Abstract: The microstructure and mechanical properties of AZ61-4Si magnesium alloy before and after equal channel angular processing (ECAP) were studied. Results show that the matrix α-Mg and divorced eutectic β-Mg17Al12 are refined and chinese script type Mg2Si phases are broken to dispersed particles after ECAP. The mechanical properties of the alloy after ECAP are significantly improved. After 4 passes of ECAP, the yield strength is increased from 50MPa to 109 MPa, tensile strength from 129MPa to 237MPa, elongation from 6% to 22%, and hardness from 61.2HBS to71.5HBS. The modification mechanism for microstructure and mechanical properties of the experimental alloy by ECAP was analyzed.

Abstract: In this work, 2.5vol. % (TiB+TiC)/Ti composite was prepared by in situ casting route then 1-D forging. The microstructure and tensile properties were presented and discussed. The results indicate that the as cast microstructure can be significantly modified by 1-D forging. After forging, TiB and TiC segregated at the prior β grain boundaries within the as-cast composite tend to fracture and align perpendicular to forging direction. Reduction in aspect ratio of reinforcements and α lath is also observed. 1-D forging can enhance the strength and elongation of as cast composite significantly. However, the increment in strength is quite limited as strain temperature increases to 700 °C. Additionally, room temperature and high temperature fracture mechanisms are also discussed.